UPDATE

Joan Collins, who stars as Alexis in ABC's "Dynasty," will be appearing on CBS next season in a six-hour miniseries, "Sins." She's cast as a French woman who climbs to the top of the fashion industry. Also starring are Marisa Berenson, James Farentino, Joseph Bologna, Capucine, Timothy Dalton and Jean-Pierre Aumont.

ABC did well in the ratings this season with "Consenting Adult," a TV movie about homosexuality, and now CBS has a movie with that theme in the works for next season. It's called "Welcome Home, Bobby" and has to do with a teen-age boy who has what the network describes as "a sexual relationship with an older man" and then must come to terms with his own sexuality. Timothy Williams portrays the young man, and Tony Lo Bianco plays his father.

Briefly noted: Ava Gardner will portray Jason Robard's mistress in NBC's remake of "The Long, Hot Summer," with Don Johnson and Cybill Shepherd. . . . Motown Productions is developing a late-night music show that King World hopes to syndicate for the fall of 1986. . . . Lee Grant has been signed to direct an "ABC Afterschool Special" for next season entitled "Cindy Eller: A Modern Fairy Tale," an updated version of "Cinderella."